In the 1970s, a population of arctic foxes on an island in the Bering Sea began to mysteriously decline. The animals were thin and mangy, and nearly all the cubs died. Today, only about 100 foxes remain.

The animals were not felled by an infectious disease, a PLOS ONE study suggests. Instead, the foxes probably suffered from high mercury exposure as a result of eating seabirds and other marine animals.

The researchers studied arctic fox fur samples from four sources: Mednyi Island, where the population crashed; museum specimens of foxes from the Commander Islands; and two populations in Iceland. Three of the groups, including the Mednyi Island population, ate marine animals, while the fourth group mostly preyed on land animals such as mice.

Mercury levels in foxes with marine diets were almost three times higher than in foxes with an inland diet, the team found. The Mednyi Island foxes rely exclusively on marine animals for food, whereas the other two coastal groups eat land animals as well.

The team also tested the Mednyi Island foxes’ blood, skin, and feces for signs of dangerous infections. But most tests came back negative, suggesting that pathogens weren’t responsible for the population’s collapse. — Roberta Kwok | 8 May 2013